San Francisco, Stockton and Sacramento were the world's three "most polluted cities" on Friday morning due to the wildfires raging in Northern California, according to a nonprofit that looks at air-quality data https://t.co/9ZdWnb7B9k

The Air Quality Index, a measure of dangerous smoke and exhaust particulates in the air, reached 246 in Oakland and 239 in San Francisco by late Friday afternoon, well into the zone of “very unhealthy.” https://t.co/wcOgxwsQTf